As is known, the motor-driven bogie of a rail and tram car comprises a frame defined by two side members, and by two cross members spaced apart in the longitudinal travelling direction of the train. The side members support a rotary front and rear axle, each fitted with two wheels mounted to run along rails, and the cross members support two motor reducers and two brake assemblies for driving and braking the axles.
The cross members have a tubular structure, and ends extending inside respective holes in the side members and welded to the side members. When running, fatigue failure sometimes occurs at the weld connection between the tubular cross member and the side member, and is presumably caused by a combination of technological weld defects (e.g. irregular weld bead, stuck welds, solid inclusions, lack of fusion, relative positioning errors between the cross member and the hole in the side member) and anomalous stress conditions not covered in standard tests and current regulations.
Fatigue cracks, in particular, are found to substantially originate in a horizontal plane corresponding to the mid-plane of the cross members, and are therefore presumably caused by bending moments having an approximately vertical axis and so produced by fatigue loads acting longitudinally on the side members. In-service strain gauge readings actually tend to show, particularly when the train is travelling along curves, anomalous stress on the frame caused by so-called “rhombusing” of the side members of the bogie, i.e. stress caused by a longitudinal load acting on each wheel in opposite directions on the two sides of the bogie.
A need is therefore felt to reinforce the tubular cross members of existing bogies, by performing repair and/or updating work at ambient temperature, without dismantling the bogies, and with no need for machining the cross members.